1. Field of the Invention
This invention relates to Automatic Gain Control (AGC) used in communication apparatuses, recording/reproduction apparatuses, and other signal-processing apparatuses.
2. Description of the Related Art
As a related art, a prior AGC system described in Japanese Patent Laid-Open No. H10-65750 has been known.
FIG. 1 shows a first example of a prior delay AGC system. The input RF (Radio frequency) is supplied to and amplified by a variable-gain RF amplifier 13. Here, in order to simplify the explanation, the variable-gain RF amplifier 13 comprises a variable attenuator 131 and fixed-gain RF amplifier 132. The output signal of the variable-gain RF amplifier 13 is multiplied with the output sine wave from a local oscillator 14 by a mixer 15 and then converted to an IF (intermediate frequency) signal. The IF signal output from the mixer is band-limited by a SAW (surface acoustic wave) filter 16 to only a signal in a desired channel. In other words, the SAW filter 16 removes signals such as the adjacent channel signals. Here, in order to simplify the explanation, the total power gain of the fixed-gain RF amplifier 132, mixer 15 and SAW filter 16 for the desired channel is taken to be unity. The output IF signal from the SAW filter is supplied to and amplified by a variable-gain IF amplifier 17, and then is digitized by an A/D converter 18 and input to a demodulation LSI 19. The demodulation LSI 19 demodulates the input IF signal by appropriate signal processing, and together with decoding the transmission-data sequence, it performs AGC of the gain Gatt of the variable attenuator 131 and the gain Gif of the variable-gain IF amplifier such that the IF signal input to the A/D converter is at a suitable power Pref. AGC is performed as follows. First, with the gains Gatt and Gif set to proper values, the power Pif of the IF signal input to the A/D converter is calculated. The passband power Pin of the input signal of the system, or the RF signal input to the variable attenuator 131, can be calculated from Pif as shown below.Pin=Pif/(Gif×Gatt)  [Equation 1]
Control of the gains Gif and Gatt is performed using the equation below based on the calculated Pin.Pin<Pt: Gatt=1, Gif=Pref/Pin  [Equation 2]Pin≧Pt: Gatt=Pt/Pin, Gif=Pref/Pt  [Equation 3]
That is, when Pin is less than a constant power Pt, it is possible to minimize the noise factor by maximizing the gain Gatt, and when Pin is greater than Pt, it is possible to suppress distortion in the RF stage by limiting the passing bandwidth power of the input to the fixed-gain RF amplifier 132 to Pt.
However, in the method described above, even though it may be possible to limit the power for desired channel in the RF stage, it is not possible to limit the total power for that entire bandwidth. Therefore, when the power of a signal not in the desired channel is large, there is an inconvenience in that the signal in the RF stage becomes distorted.
In order to solve this inconvenience, a method has been proposed in which a dedicated power detector is placed in the RF stage, and is used to independently control the RF gain. This is shown in FIG. 2.
In FIG. 2 there is a power detector 25 for detecting the power of the variable-gain RF amplifier. A rectifying circuit having a diode and capacitor can be used for this power detector 25. The output from the power detector 25 is averaged by a low-pass filter 26 in order to reduce the effects of noise and short-term power fluctuations. The average power value, which is the output from the low-pass filter 26, is input to a comparator 29. The other input to the comparator 29 is set to a reference power value (REFERENCE), which will be used as a comparison. In other words, when the average power value that is input is greater than the reference power value, the gain of the variable-gain RF amplifier 20 becomes smaller according to the output from the comparator 29, and when the average power value that is input is less than the reference power value, the gain of the variable-gain RF amplifier 20 becomes larger according to the output from the comparator 29. That is, the gain of the variable-gain RF amplifier 20 is controlled such that the average power value is equal to the reference power value.
On the other hand, a demodulation LSI 28 controls the gain of the variable-gain IF amplifier 24 such that the input power to the A/D converter 27 is a proper value.
By independently performing AGC for the RF stage and IF stage, it is possible to avoid the inconvenience of delayed AGC as shown in FIG. 1 of the signal becoming distorted in the RF stage when the power of the signal not in the desired channel is large.
However, in the case shown in FIG. 2, control of the RF gain and the IF gain is performed independently, so an inconvenience occurs that total AGC for the overall system cannot be performed.
Also, since it is not possible to control the gain directly in the RF stage from the demodulation LSI, an inconvenience occurs that it is not possible to perform adaptive control of the control speed of the RF gain control. In order to solve these inconveniences, it is necessary to input the detected power information for both the RF stage and IF stage input to the demodulation LSI, and to control both the RF gain and IF gain uniformly from the demodulation LSI based on the information.
In these prior AGC systems, data related to the total power of RF stage was not input to the demodulation LSI, so there was an inconvenience in that it was difficult to perform adjustment and adaptive control for the AGC as an overall system.